Composition and devices for gas sorption and process for their manufacturing

ABSTRACT

Compositions and devices for gas sorption are provided that can be activated just before their use, which hence do not need storing under inert atmosphere during storage and transportation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/IT2004/000053, filed Feb. 11, 2004, which was published in theEnglish language on Aug. 26, 2004, under International Publication No.WO 2004/072604 A2 and the disclosure of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

The present invention relates to a composition for gas sorption, as wellas to devices formed with the composition; the invention also refers toprocesses for obtaining the composition.

In particular, the invention refers to compositions and devices forsorption of traces of gases, both residual gases in evacuated chambersand impurities in inert gases, in applications in which the spaceavailable for the sorbing device is reduced.

Examples of systems requiring the control of the internal atmosphere,but having only limited room for the gas sorbing device, are flat paneldisplays, both field emission displays (also known as FEDs, generallyrequiring an inner vacuum atmosphere) and plasma displays (also known asPDPs, inside which there is an atmosphere of a given composition).Another particularly important application is represented by the OLED(Organic Light Emitting Display) screens, in which sorption of moisturefrom the screen inner space is essential, but it can be necessary alsoto sorb other gases, such as oxygen or carbon dioxide. Due to therelevance of this application, in the following description referencewill be made to these screens, but it is intended that compositions andsorbing devices of the invention can be used in a wide rangeapplications needing gas sorption, particularly when the thicknessavailable for housing the sorbing device is small.

OLED screens are used as displays for mobile phones, for Hi-Fiequipment, for car dashboards and the like. These kinds of screens andthe problems related to water entering inside them are disclosed, forexample, in U.S. Pat. No. 5,882,761.

The search publication RD 430061 and International Patent ApplicationPublication WO-A-02/27812 of SAES Getters S.p.A. disclosemoisture-sorbing systems which are specific for OLEDs. These systems areformed of a part with a central cavity filled with drying material, andwelded at the edges to a water-permeable membrane.

U.S. Pat. Nos. 4,985,296 and 5,078,909 and International PatentApplication Publication WO-A-01/88041 disclose sorbing systems based onan alternative approach to the problem, namely powders of the materialactive in gas sorption are incorporated in a resin or in a polymericmaterial, having gas permeability sufficient to allow gases to reach theparticles of sorbing material.

A problem common to all the known sorbing systems is nevertheless thatthese, once manufactured, must be kept under inert atmosphere,throughout storage time and during transportation, to avoid in thesesteps any contact with reactive gases and hence their loss offunctionality.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a composition andsorbing devices obtained using this composition which avoid the aboveproblem.

This object is achieved according to the present invention by acomposition comprising a gas sorbing component A dispersed in acomponent B, the latter being in the form of a cross-linked and porousmatrix, the composition being obtainable by a process in which:

-   -   at least one material A′, that can be converted through a        thermal treatment into the sorbing component A, is mixed with a        component B or a precursor B′ thereof, wherein B is chosen for        mixing with A′ if B is capable of resisting essentially        unaltered the thermal treatment for changing A′ into A, while B′        is chosen for mixing with A′ if B′ is able to transform into B        after the thermal treatment; and    -   the so obtained mixture is subjected to a thermal treatment        under vacuum or under an atmosphere of a gas inert as regards        component A, in conditions such as to cause the conversion of        material A′ into sorbing component A.

As used above with reference to component B, “essentially unaltered”means that this component either resists unchanged the thermaltreatment, or, if it undergoes modifications during the treatment, theseare such that characteristics indicated for B, namely porosity andcross-linking, are retained.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe invention, will be better understood when read in conjunction withthe appended drawings. For the purpose of illustrating the invention,there are shown in the drawings embodiments which are presentlypreferred. It should be understood, however, that the invention is notlimited to the precise arrangements and instrumentalities shown. In thedrawings:

FIG. 1 is a graph plotting the weight increase Δp versus time to showthe result of a moisture sorption test from a device made from acomposition of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention differs from all known systems because the activecomponent A is not added as such to a matrix (e.g., polymeric) duringthe first steps of system manufacturing, but it is mixed into the matrix(or a precursor thereof) in the form of a precursor A′ thereof, which isonly subsequently converted into active component A, for example justbefore using in the device of final desired. In this way, thecomposition of the invention is inactive for gas sorbing throughoutstorage and transportation steps, and consequently in these stepsparticular precautions, such as storage in containers sealed undervacuum or under inert atmosphere, are not needed.

Material A′, sorbing component precursor in the composition of theinvention, can be selected among any materials that may be convertedinto a component A capable of gas sorption by a treatment at not toohigh temperature, or in not too onerous conditions. The treatmentconditions are fixed by component B (or material B′), and by the supportonto which the composition is laid. In general, these conditions must besuch as not to destroy the cross-linked structure of B or close itspores, and not to destroy the support.

In case component A is a moisture sorber, this could be analkaline-earth metal oxide, in particular selected among calcium oxide,CaO, strontium oxide, SrO, and barium oxide, BaO. In this case,preferred precursor materials A′ are the related hydroxides, Ca(OH)₂,Sr(OH)₂ and Ba(OH)₂ respectively, that can be dehydrated to yield theoxides by treatments at temperatures from about 300 to 650° C. over afew hours, under vacuum or under inert atmosphere (the dehydration ofCa(OH)₂ can be carried out under gas, while Sr(OH)₂ and Ba(OH)₂ requirevacuum). If dehydration is performed under an inert gas, this ispreferably flowing. Among these oxides, BaO presents the best featuresin moisture sorption, but the use of CaO is preferred owing to low priceof its precursor and because, unlike BaO, it does not present toxicityproblems. Transformation of Ca(OH)₂ into CaO occurs successfully, inabout 2 hours, at a temperature of about 300° C. under vacuum. Decreaseof treatment times is also possible by increasing temperature, forexample up to about 500° C. Calcium oxide has the additional advantagethat upon moisture sorption it is changed into the hydroxide (reactionA→A′), that has the capability of sorbing carbon dioxide, CO₂. Someproducers of OLEDs have indicated that the proper functioning of thesedevices also requires the sorption of this gas. Another possiblemoisture sorber is boron oxide, B₂O₃, obtainable in a particularlyactive form from boric acid, H₃BO₃, according to the proceduresdisclosed in U.S. Pat. No. 6,304,367 (treatment at temperatures lowerthan 200° C., preferably at low pressure).

When the gas to be sorbed is oxygen, component A can be, for example, atransition metal oxide in which the metal has a low oxidation state andis easily further oxidizable. An oxide of this type is, for example,manganese (II) oxide, MnO, that easily sorbs oxygen changing into Mn₂O₃A precursor A′ suited for obtaining MnO is manganese carbonate, MnCO₃,that is quickly converted into the oxide by a thermal treatment at about300° C. under vacuum. Other useful components A for the sorption ofoxygen are FeO and Ni in finely dispersed form, that can be obtainedthrough thermal decomposition under vacuum of the correspondingoxalates, respectively FeC₂O₄ and NiC₂O₄, at temperatures in the range350-400° C. In case A is a metal such as Ni, the dispersion may befavored by the fact that decomposition takes place in the matrix B orwhile this is formed, so the metal forms “islands” that deposit on theinternal surfaces of the matrix, or small metallic aggregates that aretrapped by the same matrix.

The above cited components A are all chemical sorbers, namely, they sorbmoisture through a chemical reaction that transforms A into anothercompound. However, the invention is not limited to chemical gas sorbers,and A can also be a physical sorber, namely, a material that fixes gasmolecules at its surface by adsorption. In this case too, carrying outthe thermal conversion of A into A′ in a matrix helps in obtainingmaterial A in a finely divided form, that ensures good sorptionproperties. An example of a possible physical sorber (component A) isγ-alumina, that can be obtained by thermal decomposition under vacuum ataround 300° C. of norstrandite, a form of aluminum hydroxide, Al(OH)₃,or by thermal decomposition under vacuum at around 250° C. of boehmite,a form of aluminum oxo-hydroxide, AlOOH.

Obviously, compositions of the invention can comprise several A-typecomponents, and therefore be obtained from mixtures comprising severalA′-type materials. For example, in an OLED application, besidesmoisture, a damaging effect due to oxygen seems to be present, and inthis case a composition obtained from a mixture comprising Ca(OH)₂ andMnCO₃ can be used. In these cases, the thermal treatment conditions towhich the mixture must be subjected, must be chosen in order to ensureeffective conversion of all materials A′ into related sorbing componentsA.

In the starting mixture, materials A′ are generally used in the form ofpowders. These have preferably particle sizes lower than about 100 μm,and more preferably lower than 50 μm and even more preferably lower than30 μm. During thermal treatment for converting A′ into A, powdersobviously also undergo a morphologic transformation, and it is observedthat granule sizes slightly decrease, while essentially keeping the sameorder of magnitude. Accordingly, materials A′ must be used as finepowders to easily ensure, first of all, their homogeneous dispersion infinal composition and high surface area of component A (decisive forobtaining good properties in gas sorption), and moreover, to allowachievement of devices having a thin thickness from these compositions.

Component B and material B′ are such that, during thermal treatment forconverting A′ into A, they respectively either retain cross-linkedstructure and porosity or transform into a B-type component. B componenthas inner cohesion and is capable of retaining particles of A. Thepreferred component B or material B′ may be certain silicone resinsresistant to high temperature treatments, for example used as carriersof heat-resistant paints for mufflers, boilers or similar applications.Phenyl-methyl-polysiloxane resins, supplied by Tego Chemie ServiceItalia of Pandino (CR) Italy as Silikophen® (registered trademark ofGoldschmidt AG of Germany), are examples. These resins are resistant incontinuous treatment, by themselves, at temperatures up to 350° C., andup to about 650° C. when filled with inorganic materials. Determiningchemical composition or structure of these resins is extremelycomplicated, especially after thermal treatment. However, IR analysesshowed that, after thermal treatments necessary for converting A′ intoA, these resins lose only partially their organic components and undergopartial vitrification. In these treatments, resins used in compositionsof the invention keep cross-linking, as shown by particle retention evenafter treatment, and keep adhesion to the substrate even if wrappedaround objects with a circular section having a diameter of about 1 cm.

In the mixtures used to manufacture compositions of the invention, theweight percent of A can vary within wide limits, depending on specificweight of A′. In case of mixtures of Ca(OH)₂ in Silikophen®, initialfilling up to about 50% can be reached, obtaining after thermaltreatment a homogenous sorbing composition, as experimentally proved.

Adding other components to the mixture comprising A′ and B (or B′) isalso possible. For example, it is possible to add a solvent, such asalcohols, glycols, polyethers, and esters, useful for controllingmixture viscosity and consequently the possibility of obtaining ahomogenous deposit. Otherwise, a dispersing surface-active additive, isuseful for allowing a homogenous distribution of particles of A′ in themixture.

The compositions of the invention are generally used in the form offilms on a support. The support may be a wall of the device in whichvacuum must be maintained (or in which the inner gas composition must becontrolled). For instance, in the case of OLEDs, the support may be theinner surface of the rear side (referred to in the field as thebackplate), that may be made of glass or metal. Similarly, in the caseof FEDs or PDPs, it may be possible to deposit the mixture of A′ and B(or B′) onto a surface of one of the two glass plates making up thedisplay, preferably the rear one, and treat the assembly at thetemperature needed for the conversion A′→A. If B or B′ is a siliconeresin, this maintains good adhesion to the glass surface. Alternatively,it is possible to manufacture discrete sorbing devices, depositing theprecursor mixture on an additional support and then adhering this ontoan available wall of the final desired device (for example, on theinternal surface of the backplate of an OLED). In case of use of anadditional support, again this may be made of glass or be a metallicsheet, for example steel or Nichrofer (an alloy of nickel, chromium andiron), having a thickness, for example, of about 50 μm or less. In thecase of the additional support, it is either possible to manufacturesorbing devices one by one, or to manufacture large-sized supports(foils, sheets, plates, or tapes) coated with the precursors mixture,and then to cut these to desired size.

Precursors mixtures can be deposited on the desired support in severalways, for example through brush work or spraying. Preferred is the useof screen-printing, well known in the printing field, allowing greatercontrol of the deposit thickness (by control of screen thickness,through which the mixture is forced to pass for reaching the support).

In any case, thermal treatment for converting A′ into A can be carriedout at the most suitable moment, for example immediately beforeintroducing the sorbing device into the final system. In the specificcase of CaO, it is also possible to “reactivate” the composition, incase that during storing, transporting and handling, the oxide hasundergone partial or total transformation into the hydroxide upon watersorption (due to poor control of the process, or to an exposures to airthat is either unavoidable or preferred to lower the process complexityand costs). In this case, the ease of transformation of the hydroxideback to the oxide allows this change to be carried out more than onetime before the sealing of the device finally desired of the compositionof the invention.

The compositions of the invention can be used to produce deposits havingthickness values in a wide range. The low end of this range isdetermined by technical needs only, in particular by the need of havingcomponent A in an amount sufficient for the application. In the varioustests carried out by the inventors, the lowest limit achieved for thethickness of the final composition is about 20 μm. Compositions anddevices of the invention therefore also provide the additional advantageof an easy manufacture, particularly in thin films, of thicknesses lowerthan a tenth of a millimeter, not obtainable by systems known at presentconstituted of particles dispersed in a matrix. Thick deposits can beobtained essentially with any desired thickness, but during testing theinventors have noted that thicknesses higher than about 150 μm gaveproblems of internal cohesion, forming fractures in the deposit. As aconsequence, higher thickness deposits can be formed and used if thecracking of the deposit is not a problem in the foreseen application.

The invention will be further illustrated by the following examples.These non-limiting examples show some embodiments intended to teach tothose skilled in the art the manufacture of the invention and torepresent the best way to carry out the invention.

EXAMPLE 1

43.3 g of Ca(OH)₂, 10.3 g of dipropyleneglycolmethylether as a viscosityregulator, 1 g of Dispers 630 (a dispersing surface-active agent), 1 gof Airex 935 as a mixture deaerator, and 44.4 g of Silikophene® 80/MPAresin (Dispers, Airex, and Silikophen are supplied by Tego ChemieService Italia) are mixed under stirring. The mixture so obtained isdeposited using screen-printing on a Nichrofer sheet having a thicknessof 50 μm. From this sheet, some strips of 3×6 cm are cut. Three of thesestrips are introduced into a chamber provided with a gastight valve.This chamber, with the valve open, is introduced into an oven and thestrips are subjected to the following thermal treatment under vacuum:

-   -   heating from room temperature to 420° C. in one hour;    -   keeping at 420° C. for one hour;    -   cooling under vacuum down to 250° C. and subsequently under        argon flow down to room temperature.

The valve of the chamber containing the strips is closed, and thechamber is extracted from the oven, introduced in a glove box, opened,and then the strips are withdrawn. Inside the glove box, the depositthickness is measured using a comparator, and the results equal to 60±10μm.

Still inside the glove box, a strip is punched obtainingrectangular-shaped pieces of 10×18 mm. Two of these pieces are insertedinto a glass bulb provided with insulating valves, used to carry piecesto the system for measuring moisture sorption properties.

EXAMPLE 2

The bulb with pieces prepared as described in Example 1 is assembled ona bench for sorption tests, allowing the dosing of known amounts ofwater vapor. The bulb valve is opened, the system is evacuated to removeargon, and the pieces are connected to the system of water dosing inorder to keep a constant water pressure of 1 Pa on the pieces andmeasuring the pieces weight increase Δp in time (normalized as regardsthe deposit surface). The result of test is represented in FIG. 1,showing that samples of the invention rapidly sorb moisture from thesurrounding environment.

Thanks to the fact that the compositions of the invention can bemanufactured in extremely reduced thickness, while retaining goodsorption capabilities, these compositions are suited for use in OLEDscreens, in which the available thickness for the gas sorber is alwayslimited. In particular, compositions of the invention can be usefulespecially for manufacturing OLED screens as disclosed in JapanesePatent No. 3,293,527, in the name of Fuji Electric, which, unlikeprevious OLED screens, do not have a dedicated housing for the sorbingdevice, in the form, for example, of a recess or hollow on the rearsupport of the screen, whereby the thickness available for the sorbingmaterial is further reduced. In these conditions, the possibilityoffered by the present invention to form sorbing layers havingthicknesses even of a few tens of μm allows particularly useful andadvantageous results.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

1. A composition for gas sorption comprising a gas sorbing component Adispersed in a component B, component B being in a form of across-linked and porous matrix, the composition being obtainable by aprocess in which: mixing at least one material A′, that can be convertedthrough a thermal treatment into the sorbing component A, with thecomponent B or a precursor B′ thereof, wherein B is chosen for mixingwith A′ if B is capable of resisting essentially unaltered a thermaltreatment for changing A′ into A, while B′ is chosen for mixing with A′if B′ is able to transform into B after the thermal treatment; andsubjecting the so obtained mixture to a thermal treatment under vacuumor under an atmosphere of a gas inert as regards component A, underconditions such as to cause conversion of material A′ into sorbingcomponent A.
 2. The composition according to claim 1 wherein thecomponent A comprises a moisture sorber.
 3. The composition according toclaim 2 wherein the component A comprises an oxide selected from calciumoxide, strontium oxide, barium oxide and magnesium oxide, and thematerial A′ comprises a respective hydroxide.
 4. The compositionaccording to claim 2 wherein the component A comprises boron oxide andthe material A′ comprises boric acid.
 5. The composition according toclaim 1 wherein the component A comprises an oxygen getter.
 6. Thecomposition according to claim 5 wherein the component A comprisesmanganese (II) oxide, MnO, and the material A′ comprises manganese (II)carbonate, MnCO₃.
 7. The composition according to claim 5 wherein thecomponent A comprises iron (II) oxide, FeO, and the material A′comprises iron (II) oxalate, FeC₂O₄.
 8. The composition according toclaim 5 wherein the component A comprises metallic nickel, and thematerial A′ comprises nickel (II) oxalate, NiC₂O₄.
 9. The compositionaccording to claim 1 wherein the component A comprises a physical gassorber.
 10. The composition according to claim 9 wherein the physicalgas sorber comprises a moisture sorber.
 11. The composition according toclaim 10 wherein the moisture sorber comprises γ-alumina, and the A′material comprises either the aluminum hydroxide norstrandite or thealuminum oxo-hydroxide boehmite.
 12. The composition according to claim1 wherein the material B′ or component B comprises aphenyl-methyl-polysiloxane resin.
 13. A process for obtaining acomposition according to claim 1, comprising the steps of: mixing atleast a material A′ precursor of component A and either a component B ora material B′ precursor of component B; subjecting the thus obtainedmixture to a thermal treatment under vacuum or under an atmosphere inertas regards component A, under conditions such to cause conversion of allmaterials A′ into related sorbing components A.
 14. The processaccording to claim 13 wherein the step of thermal treatment is carriedout immediately before introducing the composition into a final desireddevice.
 15. The process according to claim 13 wherein the material A′comprises an alkaline-earth metal hydroxide and the thermal treatment iscarried out at temperature from about 300 to 600° C.
 16. The processaccording to claim 13 wherein the material A′ comprises boric acid andthe thermal treatment is carried out at a temperature lower than 200° C.17. The process according to claim 13 wherein the material A′ comprisesmanganese carbonate, and the thermal treatment is carried out at atemperature of about 300° C.
 18. The process according to claim 13wherein the material A′ comprises an oxalate of iron or nickel, and thethermal treatment is carried out at a temperature in a range betweenabout 300 and 400° C.
 19. The process according to claim 13 wherein thematerial A′ comprises either the aluminum hydroxide norstrandite or thealuminum oxo-hydroxide boehmite, and the thermal treatment is carriedout at a temperature of 300° C. and 250° C., respectively.
 20. Theprocess according to claim 13 wherein the mixture comprises severalmaterials A′ and the thermal treatment is carried out at such atemperature to ensure effective conversion of all materials A′ intorelated sorbing components A.
 21. The process according to claim 13wherein the material A′ is used in a form of powders having a particlesize lower than about 100 μm.
 22. The process according to claim 13wherein the component A comprises calcium oxide and the mixturecomprises up to 50% by weight of calcium hydroxide.
 23. A gas sorbingdevice comprising a composition according to claim 1, obtained bydepositing the mixture of precursors A′ and B′ on a support by brushwork, spraying, or screen-printing, and subjecting the deposit tothermal treatment.
 24. The gas sorbing device according to claim 23wherein the support comprises a wall of a final desired device.
 25. Thegas sorbing device according to claim 23 wherein the support comprisesan additional support.
 26. The gas sorbing device according to claim 23wherein the support comprises metal or glass.
 27. The gas sorbing deviceaccording to claim 23, wherein the composition has a thickness fromabout 20 to 150 μm.
 28. An OLED screen comprising a gas sorbing deviceaccording to claim 23.